I’ve found that lights are important even in the day time! I noticed that cars can be hard to see if they don’t have their lights on, even at high noon. I figured that, if it can be so hard to see something that size when it doesn’t have lights on, I on a bicycle must be even harder to see if I don’t have lights.
A mirror is helpful, especially if you must ride alongside traffic. In addition to being on guard for opening car doors, you’ve got to be aware of what’s coming up behind you.